Note - Shops are always, always closed in Austria on Sundays.
This is not quite true; it is the general rule, but there are exceptions, though.
In touristic towns and areas shops are open on Sundays ever since. The problem in big cities, particularly Vienna, is the definition of "touristic area". Where does it start, where does it end? Who is entitled to define it? As politics, unions and businesses can't find an agreement (ever since), everything stays closed on Sundays.
In major train stations, at the airport, and in gas stations you can buy groceries and snacks.
And there is one supermarket in Vienna, at least as to my knowledge, which has the permission to open on Sundays, for whatever reason. (SPAR at the corner of Babenbergerstraße and Getreidemarkt, close to the Kunsthistorisches Museum.)